Speculation that newly declassified US intelligence will contain smoking-gun evidence.

Three years on from the start of the pandemic, the FBI - the US domestic intelligence agency - reveals it sees a lab leak in Wuhan as the most likely source of Covid. Weeks later, the US Congress passes a law requiring the country’s intelligence agencies to declassify information they have that potentially links the Wuhan Institute of Virology with Covid’s origin. What will it reveal? And, in our final episode (for now at least), what conclusions can we reach about that question that’s sparked so much controversy and acrimony: where did Covid come from?

Archive: Fox News; C-SPAN.

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

Mysterious deaths in Soviet Russia and what they might tell us about the origin of Covid.

When US intelligence agencies blamed a spate of unexplained Russian deaths in 1979 on a leak from a bioweapons facility, the Soviet government responded angrily, saying the cause was natural. A top US scientist stepped in to find the truth - and was given anything but. Does pointing the finger of blame creative a disincentive for governments to cooperate more fully? Or should information from inside authoritarian states be treated with suspicion? A Chinese insider has a set of striking revelations and someone who dismissed conspiracy theories now has one of his own.

Archive: CBS; The White House; C-SPAN; New Yorker; New York Times.

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

What really went on in that Wuhan market - and the curious case of the raccoon dog.

It’s the early hours of 31 December 2019 and a cleaning squad is moving through the narrow lanes of a large, covered market in the Chinese city of Wuhan. The Huanan Seafood Market has been linked to a number of cases of a mysterious new illness circulating in the city, suggesting that animals there might be the source. But the World Health Organization would later say there were no verified reports of live mammals for sale. So what’s the truth about the market? And why have raccoon dogs sparked yet another bitter scientific dispute?

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

The lab leak theory goes mainstream. How safe are top-security laboratories?

After the World Health Organization’s attempts to find the origin of Covid effectively rule out a lab leak, there’s a backlash. A new US president helps bring the lab leak theory into the mainstream. What are the dangers involved in the work being done in virology laboratories? How effective are their safety measures? And do the benefits of their work outweigh the risks? John visit’s a top biosafety lab to find out.

Archive: CGTN; CBS; The White House; CNN; C-SPAN.

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

A World Health Organization team heads to Wuhan. Will they find the truth?

As the first year of the Covid outbreak draws to a close, a team of international scientists chosen by the World Health Organization is preparing to visit the city where it all started to investigate the virus’s origin. But with the Chinese government closely involved in the mission, how free will they be to find the truth? And is a researcher with links to the Wuhan Institute of Virology the right person to be asking questions about a possible lab leak?

Archive: CGTN.

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

US President Donald Trump wades into the Covid origins debate.

It’s April 2020 and at the podium of the White House’s press room, Donald Trump makes a geopolitically incendiary claim: he has a high degree of confidence that the origin of the virus that causes Covid is the Wuhan Institute of Virology - a Chinese government lab. But scientists seem certain that can’t be true. So what is the evidence for both claims? And what were the scientists publicly dismissing a lab leak saying in private?

Archive: The White House; C-SPAN; CBS.

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

A team of top scientists ask where Covid came from. But can they trust data from China?

As the new virus takes hold in Wuhan, the Chinese state downplays its infectiousness and punishes a doctor who sounded the alarm. It raises the question of trust in what China and its scientists say - a question soon at the heart of a brewing political storm over Covid’s origin. Western scientists offer a response to claims the virus could’ve leaked from a lab. But are they being given the full picture?

Archive: CBS; The White House; NPR; Weibo; CGTN; Tom Cotton; CBS; MSNBC; Democracy Now!; CNN.

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

As a deadly new virus starts spreading in Wuhan, China, so do rumours about a lab there.

In the remote, jungle-covered hills of China’s far-southwestern Yunnan Province, teams of scientists have spent years intensively researching one animal: bats. The scientists are virus hunters, trying to better understand and mitigate the threat of new viruses jumping from bats to other animals and humans, potentially setting off a pandemic. Their samples of bat droppings are brought back to labs, including the Wuhan Institute of Virology. So when a new coronavirus begins killing people in that same city, questions are raised about whether the people trying to stop a pandemic could’ve accidentally triggered one.

Archive: CBS; The White House; NPR; CGTN; NBC.

Presenter: John Sudworth Series producer: Simon Maybin Editor: Richard Vadon Sound design and mix: James Beard Commissioning editor: Dan Clarke Science advice: Julian Siddle and Victoria Gill Extra production: Eva Artesona and Kathy Long Research support: Zisheng Xu and BBC Monitoring Production coordinators: Siobhan Reed, Helena Warwick-Cross, Sophie Hill, and Debbie Richford Theme and original music: Pete Cunningham, with trumpet by Joss Murray Radio 4 Editor of Editorial Standards: Roger Mahony Head of BBC News - Long Form Audio: Emma Rippon

Cover-ups, coincidences, and conspiracy theories: where did Covid come from?

John Sudworth was the BBC’s China correspondent when an unexplained pneumonia started worrying Wuhan doctors in December 2019. Since then, he’s been investigating the origin of the virus that would turn into a devastating global pandemic.

From the beginning, there have been claims of certainty. Many scientists say the virus that causes Covid came from nature - probably carried from bats to other animals, and then to humans in a Wuhan market. But an alternative possibility - that it leaked from a laboratory - has refused to go away. And other scientists claim there’s uncertainty. For them, the mystery of Covid’s origin remains unsolved.

So, where did Covid come from? For every one of the millions of lives lost, for every long sufferer, for the pain, the isolation, and the incalculable economic cost, the answer matters. It might also help us prevent another - maybe even worse - pandemic.

But it’s a debate that’s become politicised, toxic, and a bit crazy.

The series begins on Tuesday 30 May, 2023.

Presenter: John Sudworth Series Producer: Simon Maybin Editor: Richard Vadon Commissioning Editor: Dan Clarke